STK-0050 replacement for SX-780 and others

[rcs16]

I think it is best to measure across both emitter resistors, this allows you to take into account any offset voltage. Any offset in voltage will create a small imbalance in your measurements.
80mA is on the high side, it allows for a larger class "A" region and lower THD but it makes the HS/bjts run much warmer at idle. Since the original STK-0050 has no adj for bias, I am not sure what one actually reads in a SX-780, but I assume it is around 32mA.
I doubt that you will hear any difference in sound quality with the higher bias.

Once you pass the DBT test, you are good to go, you can take it off the DBT, then adjust the bias/offset. Do not adjust bias on the DBT as it will be incorrect. Also allow the unit to warm up to thermal stability, then check/adjust bias for the final time,

 

[zebulon1]

What about using this method?
Connect a speaker, apply 1 volt pk to pk to input and monitor output with a scope.
Slowly turn up the bias until the output looks like a sine wave.
The output signal goes from completely distorted to a sine wave as the bias is increased.

 

[merlynski]

When I was 'setting' bias on dbt I was looking for adjustment function and protecting from runaway. Backed off bias pots completely before going to higher AC input. After i was on full AC power with bias set to about 32mV across both emitter resistors on both Left and Right channels I let it sit idle and soak for about 50 minutes. My temperature probe read 37 deg C (98 F) on the heads of the mounting screw for Q5 on both modules. It was not running hot to touch. FWIW I re-pasted both heat spreaders inside the heatsink and made sure they were touching the sink. When I first pulled them the old paste was very uneven and was not even present in the middle, I could see where they were not touching. I removed the burs from the heatsink screw holes, peened the plates slightly so they had a bend toward the heatsink in the middle then remounted them with two flat washers under the heads of the screw so there was no protrusion on the module side of the heatsink. With the new modules greater mounting plate to heatsink area (compared to original STK-0050) there should be plenty of thermal mass and conductivity to keep them cool.
(I tend to over-engineer [and over explain] things, my OCD is showing. I guess I spent too many years re-writing Test Procedures).
I will be back in the Shop a bit later . . .
Thanks again for all your help folks!

 

[zebulon1]

I wonder about the Sil pads at times. They can get contaminated very easy with the worst case being resistive short between the collector and heatsink.

 

[merlynski]

I like mica and paste the best. When I mounted the transistors on the plates the resistance of each collector was greater than 20 Mohms to the plate.

 

[rcs16]

I got burnt by a silpad recently, so learnt you have to check case to ground always before applying power. proper torque is important too.
keratherms are claimed to be one of the better types.
I do not think mouser sells TO-3P micas, if they do, I can add them to the BOM.

zeb your method is okay, but you do need to know the actual power dissipation in the o/p stage, since it affects the heatsink temp at idle.
a way to see how bias affects THD is to measure 20KHz thd and watch it change with bias. That is what we use in simulations. 4 ohm loading as well. it does not have to be at full power. 100m,1,10W will tell you what you need to know.

 

[merlynski]

I do not think mouser sells TO-3P micas, if they do, I can add them to the BOM.

I did not find mica TO-3P insulators on Mouser. I bought the orange film insulators and used paste.

Well my SX-780 is singing right along on full AC power. Sounds pretty good on the 4" sacrificial speakers on my bench. Meters register between .01 and 1 Watt, with peaks occasionally above 1 watt. Modules are running about 39 deg C in a still ambient environment of 28 deg C. Power supplies stable at 39.5 vdc +/- 0.3 vdc. Aux input from my bench computer output.

 

[merlynski]

After playing in the first watt for about an hour the module temps have risen to 44 deg C.
I set the bias down to 15-16 mv on both channels while still warm and began playing again. Cannot hear any difference, module temps rose again to 43-44 deg C.
Wish I had a distortion analyzer . . .
The parts replacement list on this unit is 2 main power supply caps, bumped up to 10000 uf from 8000 uf, one protection relay, and the relay driver transistor. Also a few eyelets and some copper foil tape to repair damage from a previously bungled relay replacement before I got it.
I am going to button it up and call it good .
Thanks once more to the great people on AK.

 

[petehall347]

one or both emitter resistors

I set the bias down to 15-16 mv on both channels

?

 

[merlynski]

one or both emitter resistors

?

Both emitter resistors. I made little loops on the wire from the modules to the main pcb to clip the grabbers on so I could work from the bottom side. See picture of yellow grabbers in post #457

 

[rcs16]

Good job, another satisfied customer
15-16mV across the emitters / 2 x 0.22 ohm emitters resistors = 36.4mA
I would not worry too much about measuring thd, it is just a number In all practical purposes it is non-
existent

 

[Super Noob]

This unit went toes months back... it is was cheap and gifted to a family member.

It sounded nice. Just relamped it and re-pasted the STKs.

Once I get it back (600 miles away now) - I am going to do this mod. The 2010 schematic showed STK-105 (but the unit had STK-080).

This set has some descent components - example toroidal transformer.

Anyways - thanks for the all work done and the creative minds doing this discrete component mod.

 

[Rational35]

A great-big Thank-You!! to rcs16 for taking the initiative to create this solution and share it.

I have a buddy with an SX-780: I've replaced the output modules for him with the Lxxxxxx Elecxxxxxxx "STK-0050" modules (twice), and they are a frail, pale substitute.
Failed again. He likes to rock. But they are sub-standard.

So, I just sent the STK-0050-8.zip file to PCBWay. Approved and submitted an order for 10 boards. With the New Customer coupon and all, it was only about CAD$1.08 per board, slow-shipped via China Post. (10.81 total). Man, that is just amazing to me. I'm an old-school PCB designer, and the last time I ordered PCBs, <cough> years ago, from a Canadian fab, it was $500 for a 10-piece proto lot of any PCB from 1 square inch to 1 square foot... now I can order boards for some little projects without breaking the budget, what a concept.

I really like the non-pedantic, non-preaching sharing spirit here on AK, and the whole general atmosphere. RIP Zilch.

When I get the boards, I'll let you know how it all works out. I may have some extra boards to give away.

Oh, btw, I've always used CHO-THERM 1671 thermal/isolation pads, mostly because it is very resistant to punch-through, and very reliable when you are isolating 500V switching MOSFETS. Plus, it was the standard material in use by a US military supplier I worked with in California back in the early '90s. I think I still have a 8" x 11" sheet or so of it here- I cut my own shapes and punch the holes. I've always thought it had good thermal performance, too, but I guess that assumption was based on faith and tradition. This test of thermal pad materials opened my eyes a bit: https://tfaws.nasa.gov/TFAWS03/Data/Thermal%20Session/Glasgow.pdf

FWIW. Cheers and Thanks again to rcs16.

-Peter

 

[rcs16]

Hi Peter,

Thanks, it is great that we can extend the life of these beautiful old receivers. have to thank the team as well, Hbrown for the design/proto help and Zeb for making the plates/pcbs available to the masses.

There was a discussion on diyaudio about making your own pcb's, Waste of time,$,chemicals these days making your own pcb's in the garage, one member suggested https://www.jlcpcb.com// $2.61 for 10 = amazing deal for a prototype imo. I have not used these guys. better to stick with pcbway, since we know that works well.
The Asian's have a efficient & low cost system to minimize the engineering of a production panel, handling expenses. On one design I asked for the default HASL plating ( always for hand soldering) and got Immersion gold plating, I was okay with that. it just depends what designs they are panelizing at the time, someone else paid for the extra cost of ENIG plating for me, nice people.

Good luck with your 780 repairs. make sure you use the 200W ONSEMI output bjt devices, as on the Mouser BOM, for your buddy. I challenge your buddy to blow these devices up
CHO-THERM® 1671, nice product, wish it was easy to get for us. Use the best thermal insulators you can find or have on hand. If we can find better ones at Mouser, let me know and I can add it.Mouser BOM. Silpads are convenient for this app since they are cheap and cut to size for us.

Rick

 

[Rational35]

Hi Peter,

Thanks, it is great that we can extend the life of these beautiful old receivers. have to thank the team as well, Hbrown for the design/proto help and Zeb for making the plates/pcbs available to the masses.

<snip>

CHO-THERM® 1671, nice product, wish it was easy to get for us. Use the best thermal insulators you can find or have on hand. If we can find better ones at Mouser, let me know and I can add it.Mouser BOM. Silpads are convenient for this app since they are cheap and cut to size for us.

Rick

Yes, thanks to Hbrown, Zeb, everyone else who has contributed their input, and I think punchback provided a nice dimensioned drawing of the heat-spreader plate
I have some .25" aluminum material handy, so I'll start fabricating some plates while I wait for pcbs to arrive on a slow boat from China.

About the CHO-THERM® 1671: yes it's nice, but the thing I noticed in the NASA test of thermal materials in the pdf link I posted (above in post 473), is that the Laird T-pli 220 material blows it out of the water on sheer thermal performance, by a factor of 5: Delta-T of 6.1°F (T-pli 220) vs 33.3°F (1671) at 10 in-lb of torque. The main benefit of this would be tighter thermal coupling between the BD139 and the output devices. <Edit> note: the SIL-Pad K-10 had a Delta-T of 21.8°F in the same test. /<Edit>

Mouser sells T-pli 220, but only in 8"x8" (or 16"x16") sheets. Here, from mouser.ca:

739-A10080-01 Tpli 220 A0 8x8" No Adhesive 1: $56.17 (CAD) (my choice, don't like adhesive)
739-A10080-02 Tpli 220 A1 8x8" Adhesive One side 1: $63.12 (CAD)
739-A10093-02 Tpli 220FG A1 8 x 8" Adhesive One side, Fiberglass reinforced 1: $63.12 CAD)

So, under $1 per square inch, an 8x8" sheet would be a lifetime supply for many of us. Pre-cut/punched shapes are convenient, but here is my approach to working with sheet material:

Cut off the required shape with sharp scissors. Leave a good margin- at least 1/8" all around.
Center the device on the cut material, over a piece of wood or conductive foam, and mark the mounting hole(s) with an awl.
Punch the awl through the material, and use that to center it over the tapped mounting hole.
Hold the material in place, remove the awl, and place your mounting screw over the hole.
A quick tap on the screwdriver handle to set the screw in place, then drive it in.
The screw will "die-cut" a perfect hole.

Cheers,
Peter

 

[rcs16]

The main benefit of this would be tighter thermal coupling between the BD139 and the output devices.

I just realized that the BD139 is in a TO-126 isolated package, it does not need an insulator, just some thermal compound.
if you use 863-MJF15030G and 863-MJF15031G you do not need insulators either, just some thermal compound.

 

[Luissx780]

A great-big Thank-You!! to rcs16 for taking the initiative to create this solution and share it.

I have a buddy with an SX-780: I've replaced the output modules for him with the Lxxxxxx Elecxxxxxxx "STK-0050" modules (twice), and they are a frail, pale substitute.
Failed again. He likes to rock. But they are sub-standard.

So, I just sent the STK-0050-8.zip file to PCBWay. Approved and submitted an order for 10 boards. With the New Customer coupon and all, it was only about CAD$1.08 per board, slow-shipped via China Post. (10.81 total). Man, that is just amazing to me. I'm an old-school PCB designer, and the last time I ordered PCBs, <cough> years ago, from a Canadian fab, it was $500 for a 10-piece proto lot of any PCB from 1 square inch to 1 square foot... now I can order boards for some little projects without breaking the budget, what a concept.

I really like the non-pedantic, non-preaching sharing spirit here on AK, and the whole general atmosphere. RIP Zilch.

When I get the boards, I'll let you know how it all works out. I may have some extra boards to give away.

Oh, btw, I've always used CHO-THERM 1671 thermal/isolation pads, mostly because it is very resistant to punch-through, and very reliable when you are isolating 500V switching MOSFETS. Plus, it was the standard material in use by a US military supplier I worked with in California back in the early '90s. I think I still have a 8" x 11" sheet or so of it here- I cut my own shapes and punch the holes. I've always thought it had good thermal performance, too, but I guess that assumption was based on faith and tradition. This test of thermal pad materials opened my eyes a bit: https://tfaws.nasa.gov/TFAWS03/Data/Thermal%20Session/Glasgow.pdf

FWIW. Cheers and Thanks again to rcs16.

-Peter

Hi Peter,

I have just recently found out that one of my STK 0050s are toast. Would you suggest ordering these from PCBWay instead of procuring potentially bad 0050 units on ebay? Where can I find this file? Do i simply send it to PCBWay and they assemble it? I'm sorry for all the questions, I'm new to the hobby and discouraged by the dead darlington.

 

[rcs16]

Welcome,

Take some time and read the first post plus some others that follow for pictures. They should answer your questions. you are required to assemble/order parts for this project.

 

[Rational35]

Hi Peter,

I have just recently found out that one of my STK 0050s are toast. Would you suggest ordering these from PCBWay instead of procuring potentially bad 0050 units on ebay? Where can I find this file? Do i simply send it to PCBWay and they assemble it? I'm sorry for all the questions, I'm new to the hobby and discouraged by the dead darlington.

IMHO (from experience) avoid the ebay STK0050 , they are fragile firecrackers.

I have 10 boards on order from PCBway, they have been shipped and are on their way via China Post. I only intend to keep four of them, so I will have 3 pairs of bare boards, surplus to my needs. It might still be a few weeks before they arrive here.

Here's what I'll offer:
I will mail one pair of bare boards to each of the first three AK *subscribers* who request them. No charge, postage paid. The only requirement is that you are an AK *subscriber*. That's, what, $25/year to help support this fine forum.
Caveats:
1. You will have to buy the components on the Mouser BOM and assemble/solder up the board.
2. You will have to supply/fabricate your own aluminum heat spreader/mounting plate.

Read from post #1 in this thread and all will be revealed, as suggested by Rick.

I might make up a few extra aluminum plates, not sure yet.

-Peter

 

[merlynski]

IMHO (from experience) avoid the ebay STK0050 , they are fragile firecrackers.

I have 10 boards on order from PCBway, they have been shipped and are on their way via China Post. I only intend to keep four of them, so I will have 3 pairs of bare boards, surplus to my needs. It might still be a few weeks before they arrive here.

Here's what I'll offer:
I will mail one pair of bare boards to each of the first three AK *subscribers* who request them. No charge, postage paid. The only requirement is that you are an AK *subscriber*. That's, what, $25/year to help support this fine forum.
Caveats:
1. You will have to buy the components on the Mouser BOM and assemble/solder up the board.
2. You will have to supply/fabricate your own aluminum heat spreader/mounting plate.

Read from post #1 in this thread and all will be revealed, as suggested by Rick.

I might make up a few extra aluminum plates, not sure yet.

-Peter

Please consider my request, Thanks, merlyn